1. Field of the Invention
The present invention relates to communication control, such as protocol driver control, carried out by a terminal unit connected to a local area network, and, move particularly, to a protocol driver controller and a protocol driver control method employed by a portable terminal unit driven by a battery.
2. Description of the Related Art
A conventional terminal unit carries out protocol driver control, such as file transfer control, without regard to whether or not the terminal unit is connected to a transmission line of a local area network (LAN).
The terminal unit is designed to operate continuously once it is connected to the LAN transmission line and is provided with no measure to deal with disconnection from the transmission line during operation.
The terminal unit has a display panel to display communication conditions so that a user may know to some extent if an abnormality, such as breakage of a cable to connect the terminal unit to the LAN transmission line, occurs. In this case, the terminal unit continuously loads and executes a program to communicate with the LAN irrespective of the trouble in the transmission line because it has no means to test it.
Portable terminal units driven by a battery are widely used. To communicate with a host apparatus connected to the LAN, they are connected to transmission lines of the LAN. When no communication is required, they are disconnected from the transmission lines and are used indoors and outdoors to enter data.
These terminal units start to communicate with the host apparatus without regard to whether or not they are able to communicate. Accordingly, the terminal units will uselessly control protocol drivers, if no connection is established between them and the host apparatus through the LAN transmission lines.
The portable terminal units have a display panel of limited size to display a limited quantity of information compared with stationary type terminal units. Accordingly, the portable terminal units are mainly used simply to transmit entered data to the host apparatus. In this case, it is required to automatically transmit predetermined data from the portable terminal units to the host apparatus.
The conventional terminal units, however, are incapable of confirming whether or not they are connected to the LAN transmission lines when automatically transferring data to the host apparatus.
If the batteries in the portable terminal units become exhausted, communication with the host apparatus will be interrupted and data stored in the terminal units will be lost. It is necessary, therefore, to connect the portable terminal units to external power sources and to secure a power supply before communicating with the host apparatus through the LAN transmission line.
The conventional portable terminal units, however, have no means to determine whether or not they are connected to the external power sources and, therefore, risk communication failure due to a shortage of battery power.